Currently known solutions related to proximity fuzes for airborne targets may be divided into two classes according to whether there is or is not more or less cylindrical symmetry of the sensor compared to the line of sight.
Non-symmetrical sensors, exclusively adopted on rotating carriers (spin stabilized shells or rolling missiles), because of their construction, cannot use other than time and/or spectral filtering for the rejection of false spatial and angular fixed signals, since the sensor field of view monitors different areas of the field continuously due to the rotation required to cover all possible intercept angles. Symmetrical layer-type sensors have a F.O.V. between two angles, Alfa 1 and Alfa 2 in reference to their axis.
Such sensors do not present any fixed-angle (very far) target rejection problem, unless the angle is close to Alfa 1 and/or Alfa 2 and unless the carrier precession and nutation movements move the source in and out of the field of view.
The rejection of relatively far fixed-space targets, however, is nonetheless impossible to achieve due to the fact that the detected signal duration depends on relative speed, on the cross range and on target dimensions.
Open field proximity sensors are symmetric sensors similar to the layer type, but characterized by one single Alfa 1 angle (Alfa 2 may be considered tending to 0), and they exploit, for detection purposes, the peculiar shape of a closing-in target IR signal (a saw-tooth with a slow risetime and sharp flyback).
False alarms due to fixed angle sources can thus be eliminated, but sensitivity to relatively far and sufficiently radiating fixed space sources remains a problem.
Therefore, it would be highly advantageous to provide a system which can effectively target while rejecting false IR signals from both fixed angle sources and spatially fixed sources.